Means for reducing eddy current heating of a tank in electrical apparatus

ABSTRACT

Eddy current heating of a tank wall is reduced by providing the tank wall with (1) shorting bars forming a secondary loop around a portion of the tank wall and (2) a highly laminated core of high permeability material surrounding the tank wall and in such a position that the core is encircled by said secondary loop. The shorting bar outside the tank wall is spaced therefrom to provide thermal insulation between this bar and the tank wall.

United States Patent 1 1 Deno [ 1 MEANS FOR REDUCING EDDY CURRENT HEATING OF A TANK IN ELECTRICAL APPARATUS [75] Inventor: Don W. Deno, Berwyn, Pa. [73] Assignee: General Electric Company [22] Filed: March 3, I972 211 Appl. No.: 231,519

[52] U.S. Cl ..174/35 CE, 200/147 R, 336/73, 336/84 [51] Int. Cl ..H0lf 27/28, l-l05k 9/00 [58] Field of Search...l74/35 CE; 200/147 R,'l47 A, 200/147 B; 336/84, 83, 73

[56] References Cited UNITED STATES PATENTS 1/1944 Sauer ..174/35 CE X 1 Jan. 16, 1973 Stevens ..336/73 Kramer ..174/35 CE X Primary Examiner-Darrell L. Clay Attorney-Frank L. Neuhauser et a1.

[57] ABSTRACT Eddy current heating of a tank wall is reduced by providing the tank wall with (I) shorting bars forming a secondary loop around a portion of the tank wall and (2) a highly laminated core of high permeability material surrounding the tank wall and in such a position that the core is encircled by said secondary loop. The shorting bar outside the tank wall is spaced therefrom to provide thermal insulation between this bar and the tank wall.

4 Claims, 2 Drawing Figures BACKGROUND This invention relates to electrical apparatus comprising a steel tank and a high current electrical conductor extending through the tank and, more particularly, relates to means fore reducing the extent to which said tank is heated by eddy currents induced therein by current through the conductor.

The following references are of interest with respect to the reduction of eddy current heating: US. Pat. No. 2,933,55l-Kramer; and British Pat. Nos. 265,966 (1927) and 4l6,564(1932).

One heretofore-used way of reducing eddy current heating of such a tank is to provide the tank with inner and outer shorting bars of high conductivity metal which are connected together at their ends to form a loop around a portion of the tank wall. Current induced in this loop by current in the primary conductor develops a magnetic field in the tank wall that tends to cancel the magnetic field produced by the primary conductor current.

Unfortunately, prior arrangements embodying this approach have not reduced tank heating to the extent required for certain high current applications. A significant portion of the magnetic field from the primary conductor current has escaped cancellation and had produced undue eddy-current heating of the tank. My studies of this problem indicate that the current in the shorting bar outside the tank is substantially lower than the current in the inside shorting bar, indicating that a substantial portion of the current passing through the inner shorting bar is finding a return path through the tank wall rather than through the outer shorting bar, and this current through the tank wall is contributing to undue heating thereof.

SUMMARY An object of my invention is to provide for a more complete cancellation of the magnetic field in the tank wall than have prior shorting bar arrangements of this general type.

Another object is to force a greater portion of the current flowing through the inner shorting bar to follow a return path through the outer shorting bar rather than through the tank wall.

In carrying out my invention in one form, I provide a tank having a generally cylindrical wall portion principally of steel and a high current primary conductor extending through the tank and surrounded by said wall portion. Two shorting bars, one located outside and one located inside the wall portion, are provided. These bars extend generally longitudinally of the conductor and are connected together at their opposite ends to form a secondary loop disposed around said wall portion. A highly laminated core of high permeability material is provided surrounding the primary conductor and the cylindrical wall portion in a position between the shorting bars so that said secondary loop excircles said core. The outer shorting bar is spaced from the wall portion along at least a portion of its length to provide thermal insulation between the outer bar and said wall portion.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a sectional view through a high voltage electric circuit breaker embodying one form of the invention.

FIG. 2 is a sectional view along the line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown an electric circuit breaker 10 of the high voltage gas-blast type, such as shown, for example, in US. Pat. No. 3,009,983- Oppel, assigned to the assignee of the present invention. This circuit breaker comprises an insulating support column 9 and, atop the column, a cylindrical tank 12 of a suitable low-cost metal such as mild steel. At its opposite ends the tank 12 has ends walls containing centrally disposed openings through which high-voltage lead-in bushings 15 extend.

Each of these lead-in bushings comprise a centrally disposed conductive stud 20 and a tubular insulating housing 22 surrounding stud 20. Insulating housing 22 comprises a pair of tubular shells 24 and 25 of insulating material and a tubular intermediate portion 26 of metal disposed between the insulating shells. The parts 24, 25 and 26 of the housing 22 are clamped together in a conventional manner, such as shown, for example, in the aforesaid Oppel patent.

Surrounding each lead-in bushing and forming an integral part of tank 12 is a metal mounting ring 30. The intermediate portion 26 of the bushing fits within this ring 30 and is suitably attached thereto, as by bolting or by the fastening means shown in the aforesaid Oppel patent.

Disposed within tank 12 are two pairs of separable contacts connected in series-circuit relationship between the inner ends of studs 20. Each pair of contacts comprises a stationary contact 32 mounted on the inner end of the associated stud and a movable contact 34 pivotally mounted on a centrally disposed metal housing 36 which is fixed to tank 12. A suitable operating mechanism (not shown) within the housing 32 simultaneously actuates the movable contacts 34 in one direction to open the circuit breaker and simultaneously returns the movable contacts to their illustrated position to close the circuit breaker. The operating mechanism and the movable contacts are interconnected by a mechanical linkage schematically shown at 37.

When the circuit breaker is closed as shown, current flows therethrough between its two terminals 40 and 42 via one stud 20, one set of contacts 32, 34, mechanism housing 36, the other set of contacts 32, 34, and the other stud 20. This current path is referred to herein as the primary conductor and the current therethrough as the primary conductor current.

In certain circuit breakers this primary conductor current may be many thousands of amperes in magnitude, even on a continuous basis. The magnetic field produced by this high primary conductor current induces eddy currents in the steel tank 12 which tend to overheat the tank.

For reducing this eddy current heating of the tank wall, there are provided a pair of shorting bars 50 and 52 connected together at their opposite ends to form a loop around a portion of the tank wall. These shorting bars, which extend generally longitudinally of the primary conductor, are of a high conductivity metal, such as copper, thus assuring that the loop has a relatively low resistance. I

Adjacent ends of the shorting bars 50 and 52 of a given loop are preferably connected together through the ring 30. Preferably, the outer shorting bar 52 has a flange 53 that is suitably bolted or otherwise attached to ring 30; and the inner ring is suitably attached at its end to the ring 30.

When a high conductivity loop such as 50, 52, 30 is present around the tank wall, a secondary current is induced in the loop which develops a magnetic field in the tank wall tending to cancel out the magnetic field developed by current through the primary conductor. The extent of this cancellation is increased by providing a plurality of such loops uniformly distributed about the tank periphery, and I provide such a plurality of uniformly-distributed loops, as best seen in FIG. 2; but even with this plurality of such uniformly-distributed loops present, cancellation of the magnetic field in the tank wall is still far from complete, and objectionable eddy current losses occur when the primary current is high.

In studying this problem in prior shorting bar arrangements, I have found that substantially lower currents flow in the outer shorting bar 52 than in the inner shorting bar 50. This indicates that in such arrangements a substantial portion of the induced current passing through the inner shorting bar follows a return path through the tank wall itself, apparently concentrating near the outer surface of the tank wall. Thiscurrent through the tank wall outer surface region has caused objectionable heating under conditions of high primary conductor current.

For forcing a substantially higher percentage of the induced current passing through the inner shorting bar 50 to follow a return path through outer shorting bar 52 instead of through the wall of tank 12, I provide a highly-laminated core 60 of transformer iron. This core 60 is of an annular form and is disposed outside the tank 12 and within each of the loops 50, 52, 30 so as to be encircled by each loop. Preferably, the core is made of a ribbon of high permeability transformer iron, e.g., grain-oriented silicon steel, having a thin insulating coating thereon and wound in many layers about its central axis to provide a highly laminated structure.

The presence of this laminated core 60 results in a closer coupling between the primary conductor and the secondary shorting bars, which in turn results in more current through the outer shorting bar 52. This increased secondary current through the outer shorting bar 52 is accompanied by a decrease in the current flowing through the parallel path extending through the outer surface of the tank wall. This decrease in the tank wall current results in a decrease in the tank wall heating by such current. Another way of viewing this change is that the increased secondary current through outer shorting bar 52 provides for increased cancellation of the magnetic field in the tank wall resulting from primary conductor current and, hence, reduced tank wall heating by the eddy currents from such magnetic field.

While increased current through the outer shorting bar 52 does produce increased heating of the shorting bar 52, this increased heating is limited to a much lower value than the amount of heating produced by such current flowing through the tank wall in view of the low resistance of the copper shorting bar 52 as compared to that of the steel tank. In addition, increased heating of the shorting bar can be tolerated since it is the tank that it is desired to protect from heating. The tank is protected from such increased heating of the shorting bar 52 because the shorting bar is, in effect, thermally uncoupled or insulated from the tank in view of its being spaced from the tank wall and its being disposed outside the tank in open air. A very substantial portion of the heat generated in the shorting bar can therefore be transferred to the surrounding air without appreciably heating the tank;

For providing mechanical support for each of the outer shorting bars 52, spacers 64 are suitably attached to the tank wall 12 and the shorting bar 52 in positions between the two parts 12 and 52. The spacers 64 are preferably, though not necessarily, of a thermal insulting material.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Electric apparatus comprising:

a. a tank having a generally cylindrical wall portion principally of steel,

b.- a high current primary conductor extending through the tank and surrounded by said generally cylindrical wall portion,

. a pair of shorting members, one located outside said cylindrical wall portion and the other located inside said cylindrical wall portion,

. said members extending generally longitudinally of said conductor and connected together at their opposite ends to form a secondary loop which is disposed around said generally cylindrical wall portion of the tank,

. said shorting members being of a material having a higher electrical conductivity than the principal material of said tank wall portion,

f. a highly laminated core of high permeability material surrounding said primary conductor and said cylindrical wall portion of the tank in a position between said shorting members so that said secondary loop encircles said core,

. said outer shorting member being a shorting bar spaced from said cylindrical wall portion along at least a portion of the length of said shorting bar to provide thermal insulation between said shorting bar and said cylindrical wall portion.

2. Electrical apparatus as defined in claim 1 and further comprising: additional secondary loops, each tive stud forming a part of said primary conductor and an insulating housing surrounding said stud and supporting said stud on said tank.

4. The electric apparatus of claim 3 in which:

a. each of said openings is surrounded by a ring forming an integral part of said tank.

b. at opposite ends of said loops, said shorting members have their adjacent ends interconnected through said rings. 

1. Electric apparatus comprising: a. a tank having a generally cylindrical wall portion principally of steel, b. a high current primary conductor extending through the tank and surrounded by said generally cylindrical wall portion, c. a pair of shorting members, one located outside said cylindrical wall portion and the other located inside said cylindrical wall portion, d. said members extending generally longitudinally of said conductor and connected together at their opposite ends to form a secondary loop which is disposed around said generally cylindrical wall portion of the tank, e. said shorting members being of a material having a higher electrical conductivity than the principal material of said tank wall portion, f. a highly laminated core of high permeability material surrounding said primary conductor and said cylindrical wall portion of the tank in a position between said shorting members so that said secondary loop encircles said core, g. said outer shorting member being a shorting bar spaced from said cylindrical wall portion along at least a portion of the length of said shorting bar to provide thermal insulation between said shorting bar and said cylindrical wall portion.
 2. Electrical apparatus as defined in claim 1 and further comprising: additional secondary loops, each constructed and located as specified in claim 1, the shorting bars of said loops being circumferentially spaced-apart around the periphery of said cylindrical wall portion, with each loop encircling said cylindrical wall portion and said laminated core.
 3. The electrical apparatus of claim 1 in which: a. said tank has end walls containing openings therein, b. lead-in bushings project through said openings, c. each of said lead-in bushings comprises a conductive stud forming a part of said primary conductor and an insulating housing surrounding said stud and supporting said stud on said tank.
 4. The electric apparatus of claim 3 in which: a. each of said openings is surrounded by a ring forming an integral part of said tank. b. at opposite ends of said loops, said shorting members have their adjacent ends interconnected through said rings. 